avocado: Make each test to print its label at the beginning of each test

Fixes bug #77.

Replace the basic logging.StreamHandler used to control
avocado stdout for a version that allows us to skip printing
newlines at each message. Then use this stream handler to
log the test label at the beginning of each test, rather
than at the end. When the tests are done, they can now simply
print STATUS strings appropriately, therefore not requiring
to log the test label.

Both the HumanTestResult and VMTestResult classes were
adapted to use this new soluton.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>